1. Field of the Invention
The present invention relates generally to channel coding for wireless communication systems.
2. Description of the Related Art
Wideband Code Division Multiple Access (WCDMA), also known as Universal Mobile Telecommunication System (UMTS), is a technology for wideband digital radio communications of Internet, multimedia, video and other capacity-demanding applications. WCDMA uses a spectrum with a 5 MHz carrier, providing 50 times higher data rate than in present Global System for Mobile Communication (GSM) networks, and 10 times higher data rate than in General Packet Radio Service (GPRS) networks. WCDMA is one technology for 3G telecom systems, providing higher capacity for voice and data and higher data rates.
High Speed Downlink Packet Access (HSDPA) is a packet-based data service in the W-CDMA downlink with data transmission presently up to 8-10 Mbps (and presently up to 14.4 Mbps for MIMO systems) over a 5 MHz bandwidth in WCDMA downlink. HSDPA implementations include Adaptive Modulation and Coding (AMC), Hybrid Automatic Request (HARQ), and advanced receiver design.
HSDPA consists of a forward link data channel called a High-Speed Downlink Shared Channel (HS-DSCH). This is based on shard-channel transmission, which means that some channelization codes and the transmission power in a given cell are seen as a common resource, which is dynamically shared between users in the time and code domains. Shared channel transmission results in more efficient use of available codes and power resources.
Shared channel transmission results in more efficient use of available codes and power resources compared to the current use of a dedicated channel in WCDMA. The shared code resource onto which the HS-DSCH is mapped may consist of up to 15 codes. The actual number employed depends on the number of codes supported by the terminal/system, operator settings, desired system capacity, etc. The spreading factor (SF) is fixed at 16, and the sub-frame duration (Transmission Time Interval, TTI) is only 2 ms. The modulation schemes used are Quadrature Phase-Shift Keying (QPSK) and 16-QAM.
For HSDPA, fast link adaptation is done using adaptive modulation and coding based on the channel quality indicator (CQI) feedback, instead of power control as in WCDMA. The highest possible data-rate on a given link is ensured by link adaptation for both near users (high coding rate) and far users (low coding rate). While connected, an HSDPA user equipment (UE) periodically sends a CQI to the base station (BS) indicating the data-rate, coding and modulation scheme to be used, and the number of multi-codes the UE can support under its current radio conditions. The CQI also contains the information about the power level to be used.
Fast retransmission is done using Hybrid ARQ with incremental redundancy or chase combining. The retransmitted packets also use a different Gray coded constellation based on the bit reliabilities using the log likelihood ratio (LLR), in case of 16-QAM modulation. This modulation rearrangement improves the turbo decoding performance by averaging the bit reliabilities of QAM constellations with alphabet size greater than four. The UE also sends ACK/NACK for each packet such that the BS knows when to initiate retransmissions.
For HSPDA services, fast scheduling is done at the BS rather than at the radio network controller (RNC), as in WCDMA. This is done based on information on the channel quality, terminal capability, and quality of service (QoS) class and power/code availability. This channel-sensitive opportunistic scheduling obtains multi-user diversity gain by preferentially transmitting to users with better channel conditions.
In 3rd generation partnership project (3GPP) standards, Release 5 specifications focus on HSDPA to provide data rates up to approximately 10 Mbps to support packet-based multimedia services. MIMO systems are focused on in the Release 6 specifications and beyond, which are being developed to support even higher data transmission rates up to 14.4 Mbps. HSDPA is evolved from and backward compatible with Release 99 WCDMA systems.
Communication networks and systems adapted for HSPDA service are required to support transmission on up to fifteen (15) 16-Quadrature Amplitude Modulation (QAM) channelization codes to multiple users and/or to a single user. In a conventional example, each channel element (which may be understood herein as a given baseband processor capable of processing and transmitting digital bits of information over an air interface) is capable of supporting four (4) 16-QAM codes. Thus to support twelve (12) 16-QAM codes (for example), three (3) channel elements (CEs), e.g., baseband processors capable of processing and transmitting digital bits of information over an air interface should be used, with each CE transmitting four (4) 16-QAM codes.
Conventionally, to transmit data using all 12 QAM channelization codes to a single user, the channel encoding of a given Transmission Time Interval (TTI) of a HS-DSCH transport channel block, or “TTI block” occurs on a single CE, up to the physical channel segmentation, where after the data may be divided among multiple physical channels (PhCHs). After physical channel segmentation, the data may be sent to multiple CEs to transmit the data over the air using their corresponding 12 16-QAM transmitters.
There may be at least two potential drawbacks with the conventional approach. Firstly, channel encoding of the TTI block may lead to a longer latency, since most of the channel encoding occurs on a single CE, and then the data is typically transferred using a high-speed serial bus to multiple CEs. Secondly, use of a high-speed, inter-CE serial bus is required, which may add to system cost and complexity.